1 BASIC WAVE CONCEPTS

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1 Reading: Main 9.0, 9.1, 9.3 GEM 9.1.1, 9.1.2 Giancoli ? BASIC WAVE CONCEPTS

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L1&2(basic waves).pptBASIC WAVE CONCEPTS
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REVIEW SINGLE OSCILLATOR: The oscillation functions you’re used to describe how one quantity (position, charge, electric field, anything ...) changes with a single variable, TIME.
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time
-A
A
Oscillations in time
Angular frequency, ω, dimension: [time-1] Period, T, dimension: [time] Amplitude A, or ψ0, dimension: [whatever] Phase, ωt+φ, dimensionless Phase constant, φ, dimensionless
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Equivalent representations …….
6 If we have SEVERAL oscillators at different positions, we can describe the variation of that same quantity (call it ψ) by a function of TWO variables: TIME, just like before, and another variable, POSITION, which identifies the location of the oscillator.
Watch the animation. What can you say about the amplitude, frequency and phase of each oscillator? Which direction does each oscillator travel? Which way does the wave travel?
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-A
A
time
space
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Periodic variations in space
Wave “vector”, k, dimension: [length-1] (wave number is 1/λ) Wavelength, λ, dimension: [length] Amplitude A, or ψ0, dimension: [whatever] Phase, kx+φ, dimensionless Phase constant, φ, dimensionless
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Looking ahead…. We will discuss mostly harmonic waves where variations are sinusoidal. Traveling and standing waves Damped, (driven) waves Reflection, transmission, impedance Classical and quantum systems
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Traveling wave
Standing wave
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ψ (x,t) = Acos ωt ± kx + φ( ) Traveling waves - functions of ωt±kx
Disturbance propagates … what speed? Look at one particular feature (constant phase)
λ = 2π k ;T =
ωdt ± kdx = 0
d ωt ± kx( ) = 0 v = dx/dt or phase velocity is velocity of one particular feature. If we had ψ = Acos(ωt-kz), it would be v=dz/dt.
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ψ (x,t) = Acos ωt ± kx + φ( ) Traveling waves - functions of vt±x
vt-x disturbance travels in direction of increasing x
(t↑⇒ x↑ for constant phase) vt+x: travels in direction of decreasing x
(t↑⇒ x↓ for constant phase) Traveling waves are
superpositions of standing waves
λ = 2π k ;T =
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Focus on the green circle that marks a maximum (a particular phase)
phase velocity, vphase= ω/k =λ/T, dimensions [length. time -1]
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Another velocity - material/field velocity When a wave or disturbance propagates, the particles of the MEDIUM do not propagate, but they move a little bit from their equilibrium positions
material velocity, vmat=∂ψ/∂t, dimensions [ψ. time -1]
Material/field velocity is speed of the “waving thing” in the medium. If ψ has dimensions of length, this is a velocity as we normally think of it. But waves don’t necessarily need a medium in which to propagate and ψ might well represent something more abstract like an electric field. If you think of a better name, let me know!
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Focus on the red circle that marks a particular x
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Material/field velocity vmat=∂ψ/∂t dimensions [ψ. time -1]
If the material/field velocity is perpendicular to the phase velocity, the wave is “transverse”. Examples?
If the material/field velocity is parallel to the phase velocity, the wave is “longitudinal”. Examples? Combinations of the above are possible. Examples? Show wave machine
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Another velocity – group velocity There is another way to make something that has the dimensions of a velocity:
Group velocity, vgroup=∂ω/∂k, dimensions [length. time -1]
This describes the propagation of a feature in a “wave packet” or superposition of waves of different frequencies. We will come back to this concept later.
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Standing waves - functions of x (only) multiplied by functions of t (only)
Standing waves are superpositions of traveling waves
ψ(x, t) = Acos kx( )sin ωt( )
Also find mixtures of standing waves and traveling waves
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Other waveforms (e.g.) sawtooth, pulses etc., can be written as superpositions of harmonic waves of
different wavelengths and/or frequencies … Fourier series and Fourier integrals (transforms)




ψ(x, t) = Re Dei ωt−kx+φ( )[ ] Same conversions between A, B, C, D forms as for
oscillations - see Main Ch1, Ch9.
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This DE results when: Newton’s law is applied to a string under tension Kirchoff’s law is applied to a coaxial cable Maxwell’s equations are applied to source-free media … and many other cases …
∂ 2
∂t 2 ψ (x,t)
How do these functions arise? PROVIDED ω/k = v, a constant, they are solutions to the differential equation: (non-dispersive wave equation)
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forms of 2-variable function
BASIC WAVE CONCEPTS -REVIEW